MRI apparatus use strong magnetic fields, high power radio frequency (“RF”) energy, and rapid precise magnetic field variations to analyse properties of materials. MRI apparatus are commonly used in clinical applications to image body tissue.
MRI has also been used in other fields, notably oil and gas exploration where reservoir rock core samples are analysed to obtain information about the nature of the reservoir being investigated. In the ground, the reservoir rock can be under tremendous pressure and elevated temperatures. It is desirable to reproduce such reservoir conditions when performing tests on reservoir rock core samples. In order to do so, however, the core holder which houses the core sample must be capable of withstanding elevated pressures and temperatures as found in reservoirs.
This presents challenges as to the materials that can be used for the holder. Metal core holders are known which are capable of withstanding elevated pressures and temperatures. Metal core holders, however, block the nuclear magnetic resonance (NMR) signal in the core sample from being detected in the RF probe. In addition, the rapidly switched magnetic field gradients induct currents in the metal called eddy currents. These eddy currents distort the magnetic field and thus distort the resultant magnetic resonance image.
Hardware improvements such as shielded gradient coils and waveform pre-emphasis are largely successful at reducing these effects in modern scanners in the absence of substantial metal structures near the sample space. The residual eddy currents may however still cause image-quality problems including ghosting in EPI, RARE and GRASE imaging pulse sequences, slice-profile modulation with spatial-spectral RF pulses, geometric distortion in diffusion-weighted EPI, and quantitative velocity errors in phase-contrast imaging. Knowledge of the true gradient waveform in the MRI pulse sequence is critical to addressing and remedying such problems. Eddy currents present a particular problem in sample spaces surrounded by or in the vicinity of metals structures.
Numerous methods have been developed to measure MRI gradient waveforms and k-space trajectories. One strategy is magnetic field monitoring (MFM) with RF microprobes. Multiple RF microprobes record the magnetic field evolution associated with a wide variety of imaging pulse sequences.
The MFM method involves exciting the sample and measuring the time evolution of magnetization through the free induction decay (FID). However, the gradient waveform duration is limited by the sample T2*. The k-space maxima (i.e. maximum temporal gradient area or image resolution) measurable with MFM are also limited by gradient dephasing. In addition, implementation of this technique is relatively complex as it requires careful probe fabrication, an array of at least 3 probes, accurate probe positioning and alignment and a multi-channel receiver.
Core holders made of non-metallic composite materials are also known. Such core holders eliminate the problems associated with using metal but are limited by the pressures and temperatures that they can sustain. The sensitivity of the NMR apparatus is reduced due to the larger RF probe being required to accommodate the holder. For non-gradient NMR techniques like measurements of free induction decays, measurement of T2 and/or T1 compensation of the gradient distortion is not required.